In wireless communication, there is broadening of signal band width as a method for realizing large capacity communication. In a frequency band equal to or lower than several gigahertz, frequencies have already been allocated to a large number of systems. It is difficult to secure a wide signal band equal to or higher than 100 megahertz. In cellular communication in which a communication area covered by one radio base station is equal to or larger than several hundred meters, a frequency band equal to or lower than several gigahertz has to be used.
On the other hand, in a high frequency band of several ten gigahertz, there are a lot of unallocated frequencies, that is, free spaces. It is highly likely that wide signal band width equal to or higher than several hundred megahertz can be secured. However, in a high frequency band of several ten gigahertz, because a propagation distance attenuation amount is large, there is a disadvantage that a communication area cannot be secured wide. It has been considered to make it possible to greatly increase the number of antenna elements mountable per fixed area, form a beam having a high gain, and compensate for propagation distance attenuation making use of the fact that the length of one wavelength is short in a high frequency band.
To realize large capacity transmission, in addition to the broadening of a band, there is a technology called super-multi-element multiple-input multiple-output (MIMO) or Massive MIMO for spatially multiplexing transmission signals by forming a plurality of beams in a large number of antenna elements. In general, a communication apparatus of the super-multi-element MIMO includes a digital-signal processing unit that includes a series of a transmission circuit and a reception circuit for each of antenna elements and is adaptable to the number of antenna elements. For example, when there are 256 antenna elements in the super-multi-element MIMO, the communication apparatus needs to include a digital signal processing unit that uses 256 transmission circuits and reception circuits, an FPGA (Field Programmable Gate Array), an ASIC (Application Specific Integrated Circuit), and the like and is adaptable to the 256 elements. Therefore, there are problems in cost and viability.
As measures against the problems, there is a method of realizing beam formation with an analog circuit including a variable amplifier and a variable phase shifter rather than with the digital-signal processing unit. This is a method of forming an analog beam. Consequently, a required number of transmission circuits and reception circuits is not the same as the number of antenna elements and is the same as the number of beams to be formed. The digital-signal processing unit only has to be adaptable to the number of elements same as the number of beams to be formed. As an example, when one analog beam is formed by sixteen antenna elements, the number of transmission circuits and reception circuits can be reduced to 1/16. A technology for suppressing an increase in a circuit size in this way is disclosed in Patent Literature 1 described below.
In wireless communication in recent years, a technology called OFDM (Orthogonal Frequency Division Multiplexing) or OFDMA (Orthogonal Frequency Division Multiple Access) is widely used. The OFDM or the OFDMA allocates OFDM symbols for each of a plurality of orthogonal frequencies called subcarriers.
Cellular communication is assumed in which a network is configured from a wireless communication apparatus, which is a base station, and a plurality of terminals subordinate to the wireless communication apparatus. When transmission lines between the wireless communication apparatus and the terminals have different frequency selectivities, there is user diversity for using only a subcarrier having a high transmission line gain and sharing one OFDM symbol among a plurality of terminals to obtain a frequency diversity effect among users. The user diversity is one of most important functions in the OFDMA. When there are a plurality of terminals, beam formation is individually controlled for each of the terminals. Even if a beam to a certain terminal is received by another terminal, reception power is often low and a CNR (Carrier to Noise Ratio) is small.